The invention relates to a method of identifying glazing that has undergone a heat treatment. The term xe2x80x9cglazingxe2x80x9d encompasses single or multiple glass panes, these being bare or coated with thin films, such as pyrolytic films, or with thick films, such as enamels.
Although the invention is not limited to such applications, it will more particularly be described with reference to glazing that has undergone a treatment of the xe2x80x9cHeat Soak Testxe2x80x9d type. This type of treatment allows, in a destructive manner, the detection of nickel sulphides in glass substrates, such as toughened, semi-toughened or hardened sheets of glass.
The presence of nickel sulphides in such glass substrates is known in the literature, but their origins are not very clear. Many origins have already been envisaged; it could come from the reduction of sodium sulphates and from the reaction with nickel oxide, coming, for example, from the nozzles of burners. It turns out in fact that, at the present time, no specific origin has been able to be determined, but these nickel sulphides are present in glass in the form of inclusions. Although these inclusions do not impair the main properties desired of the glass substrates, they do nevertheless result in a major problem in the case in particular of toughened glass sheets. This is because the presence of these nickel sulphide inclusions results in so-called xe2x80x9cspontaneousxe2x80x9d breakages of toughened glass sheets well after their production. Certain catalogued cases have shown breakage of such a toughened glass sheet more than ten years after its manufacture. The consequences of this drawback are serious, since the glass sheets have, of course, already been sold and then used. Moreover, since this type of glazing is especially used for the external cladding of building facades, a breakage in the glazing after it has been installed may lead to serious accidents due to the said glazing falling from, for example, building facades or from glazed roofs covering pedestrian walkways.
The various nickel sulphide compositions causing these problems have already been demonstrated. These are especially stoichiometric nickel sulphide NiS, Ni7S8 and nickel sulphides substoichiometric in nickel, NiS(1+x) with x varying from 0 to 0.08. These various compositions may be present in the glazing in the form of crystalline inclusions, the damaging diameters of which are essentially between 40 xcexcm and 1 mm.
The abovementioned phenomenon of xe2x80x9cspontaneousxe2x80x9d breakage, but the breakage occurring over time, has also already been explained. Breakages associated with the presence of nickel sulphides in the glass are caused by the volume expansion which accompanies the transformation of the xcex1 phase (hexagonal phase) to the xcex2 phase (rhombohedral phase). The xcex1 phase is the xe2x80x9chigh temperaturexe2x80x9d phase of nickel sulphides, which is metastable at room temperature. The xcex2 phase is the xe2x80x9clow temperaturexe2x80x9d phase, which is stable at room temperature. It is thus understandable that, if nickel sulphides exist in their xcex1 phase within glass sheets ready to be used or already used, transformations to the xcex2 phase will occur over time.
The presence of nickel sulphides in their xcex1 phase within glass sheets at room temperature may be explained, especially in the case of toughened glass sheets, by the heat treatment that they have undergone; this is because, in the case of thermal toughening, the rise in temperature of the glass sheet may lead to the appearance of the xcex1 phase if nickel sulphides are present. The rapid cooling which follows does not allow, because of its speed, complete return to a xcex2 phase. The glass sheets thus treated may therefore include nickel sulphides in the xcex1 phase, which will be transformed over time to a xcex2 phase, the said transformation being accompanied by a volume increase that causes the glass sheets to break.
For some uses of such glazing, especially those in which the breakage of the said glazing results in the risk of accidents, it is therefore necessary to detect glass sheets having nickel sulphide inclusions liable to cause breakage of the glass sheets over time.
One method, widely used, for detecting nickel sulphide, called the xe2x80x9cHeat Soak Testxe2x80x9d, consists in accelerating the transformation of the high-temperature xcex1 phase to the low-temperature xcex2 phase, with respect to the rate of transformation at room temperature. Such a method therefore consists of a defined heat treatment resulting, as indicated previously, in destruction of any glazing comprising nickel sulphide inclusions.
In order to use such glass sheets, for example for the building industry, it is important to be able to identify the toughened glass sheets which have undergone a nickel sulphide detection treatment. This is because, although the building industry permits the use of toughened glass sheets which have not undergone this detection treatment, there are uses which require glass sheets free of nickel sulphide inclusions; these uses are, for example, the cladding of building facades which, above a certain height, make it dangerous to use glass sheets that may undergo spontaneous breakage over time.
The identification of glazing that has undergone a detection treatment therefore appears to be necessary in order to avoid any risk of confusion between glazing that has been treated and other glazing that has not been treated and which may, moreover, be identical. It is in fact preferable to prevent any confusion that may arise, for example when delivering or when storing the glazing.
One solution for the glassmaker consists in sticking a label on the glazing that has undergone a nickel sulphide detection treatment. However, such a solution has drawbacks; first of all, whatever the type of adhesion of the label, this may have deteriorated or even disappeared. Secondly, it requires vigilance and an infallible organization in order to prevent any error, and more particularly to prevent a label being stuck on untreated glazing. Although such an organization is achievable, it may be difficult to implement.
Thus, the objective of the invention is to provide a method of identifying treated glazing, especially treated using a technique of the xe2x80x9cHeat Soak Testxe2x80x9d type, by an indelible means under normal handling and storage conditions.
This objective is achieved according to the invention by a method of identifying glazing that has undergone a heat treatment, consisting in fixing a substance o its surface and/or onto an edge, an optical characteristic of the said substance being modified at a temperature reached during the heat treatment. In the case of glazing that includes thin or thick films, the substance may be fixed either on the glass or on he film.
According to a preferred way of implementing the invention, the optical characteristic modified is the colour. This choice makes it possible, in particular, for the modification to be very rapidly recognized usually. During manufacture or storage of the glazing, it is thus easy to avoid any risk of confusion be:ween treated glazing.
Other optical characteristics, such as light transmission or gloss, may be used.
According to the invention, a possibly coloured substance is therefore fixed to the glazing before the heat treatment, for example before the xe2x80x9cHeat Soak Testxe2x80x9d treatment in the case of nickel sulphide detection, and its optical characteristic changes during the said treatment. This method according to the invention thus permits, under the normal handling, and storage conditions, he glazing that has undergone the said heat treatment to be indelibly marked. Moreover, for the glassmaker this method simplifies the incorporation of this identification, the said identification being by its nature associated with the treatment; there is in fact no risk of marking untreated glazing.
The temperature leading to a change in the optical characteristic of the substance is, of course, above the temperatures to which the glazing may be subjected during their storage or handling at the glassmaker""s premises and/or by the user.
According to a preferred way of implementing the invention, the resulting optical characteristic, that is to say the modified optical characteristic of the substance, obtained after treatment is defined by the maximum temperature reached during the treatment. According to such a method of implementation, it is thus furthermore possible for the glassmaker to check and/or prove that the said temperature has indeed been reached during the treatment. Particularly in the case in which several panes of glazing are treated in a heated chamber, this implementation may make it possible to see and/or check that the temperature has been reached at every point in the chamber or at the very least in any place in the chamber where the panes are placed during the treatment.
According to an advantageous variant of the invention, the modification of the optical characteristic appears after a temperature hold in the treatment. According to this variant, the modification of the optical characteristic is therefore obtained when the glazing has been treated at the abovementioned temperature for a given time. In the case of a heat treatment which comprises a temperature hold, it is thus possible to stick on the glazing a substance whose variation in the optical characteristic depends, on the one hand, on the treatment temperature and, on the other hand, on the time that the glazing spends at this temperature.
Such an implementation has the advantage for the glassmaker to be able to control not only the temperature in the chamber during the treatment, but also the constancy of this temperature. Furthermore, it is thus possible for the glassmaker to provide a guarantee to his customers that the heat treatment has actually been carried out.
Also preferably, the invention provides for the resulting optical characteristic of the substance to depend on the residence time at a temperature reached during the heat treatment.
According to the latter preferred implementation of the invention, it is furthermore possible to authenticate the execution of the heat treatment. It is because the modification of the optical characteristic is defined, on the one hand, by the temperature, but also by the residence time at this temperature. Not only does too short a time not lead to the defined modification of the optical characteristic, but furthermore too long a time also leads to another modification of the optical characteristic.
According to one variant, and more particularly in the case in which the substance undergoes a modification of the optical characteristic associated with a temperature reached during the treatment but independent of the residence time at this temperature and of a step going to higher temperatures, the invention advantageously provides for the substance to be removed at the said higher temperatures. This alternative way of implementation makes it possible to avoid marking heat-treated glazing when too high a temperature is reached during the treatment, this having an inhibiting effect or even an effect contrary to the one desired.
More particularly in the case of a treatment of the xe2x80x9cHeat Soak Testxe2x80x9d type, it is known that at a temperature above approximately 330xc2x0 C., the transformation of the xcex1 phase to the xcex2 phase does not occur properly, because of the kinetics of the two transformation back reactions. It is therefore beneficial to avoid marking the glazing that has been treated at too high a temperature and for which glazing the detection of nickel sulphides, which might generate spontaneous breakage, has been unable to be carried out with certainty.
Again more particularly in the case of a treatment of the xe2x80x9cHeat Soak Testxe2x80x9d type, the substance is deposited on the glazing after a thermal toughening treatment. It is thus possible either to provide a variation in the optical characteristic, which would be independent of the xe2x80x9cHeat Soak Testxe2x80x9d treatment or to destroy the substance during the rise in temperature of the glass in order to carry out the thermal toughening.
According to a preferred variant of the invention, the substance is a curable ink. This may, for example, be ink of the epoxy, polyurethane, acrylic, or other type.
According to this latter variant, the ink is advantageously deposited on an end and/or an edge of a pane, the latter having already been thermally toughened. The ink thus deposited is then cured at a temperature below that which modifies the optical characteristic during the heat treatment. Preferably it does not exceed 220xc2x0 C., especially in the case of a treatment of the xe2x80x9cHeat Soak Testxe2x80x9d type. During a treatment of the xe2x80x9cHeat Soak Testxe2x80x9d type, the pane is raised to a temperature which may be between 270xc2x0 C. and 330xc2x0 C. At these temperatures, and possibly for a given time at a temperature, the ink degrades; this degradation is visually manifested particularly by a colour variation which confirms that a given temperature has been reached and/or that time has been spent at a given temperature.
The invention thus described therefore makes it possible to be able to identify, directly on a pane, the heat treatment that it has undergone, or indeed to prove that the said treatment has been carried out.
Further advantageous details and characteristics of the invention will emerge below from the description of an example of its implementation.
This example relates to the marking of glazing heat treated using a method of the xe2x80x9cHeat Soak Testxe2x80x9d type for the purpose of detecting nickel sulphide inclusions.
The glazing undergoes thermal toughening beforehand. During this treatment, the temperature of the glazing is raised to approximately 650xc2x0 C. During this temperature rise, if nickel sulphide inclusions are present they are transformed from the low-temperature xcex2 phase into the high-temperature xcex1 phase. The rapid cooling which follows results in the xcex1 phase, unstable at low temperatures, being frozen in. The xcex1 phase-to-xcex2 phase transformation reaction then continues, but very slowly at ambient temperatures during use of the glazing. The said transformation may take place over very long periods of time, possibly of he order of several years. Moreover, the said xcex1 phase-to-xcex2 phase transformation is accompanied by an expansion of the inclusions, and this may bring about breakage of the glazing which, when the latter is used for example as a building facade, may prove to be dangerous.
It is therefore of paramount importance for the glassmaker o deliver toughened glazing that is risk-free during its use and is therefore free of nickel sulphide inclusions.
To do this, one solution consists in detecting, and eliminating by destruction, any glazing having nickel sulphide inclusions. The treatments of the xe2x80x9cHeat Soak Testxe2x80x9d type allow, by raising the temperature, the transformation of the xcex1 phase to the xcex2 phase to be activated.
The glazing forming the subject of the test undergoes a xe2x80x9cHeat Soak Testxe2x80x9d treatment consisting of a temperature hold for a period of at least two hours at 300xc2x0 C.
The object of the present invention is to mark the glazing that has undergone this treatment. To do this, an epoxy ink is deposited, before the xe2x80x9cHeat Soak Testxe2x80x9d treatment, along the edge of a surface of the glazing using a screen-printing technique. The ink deposited is sold under the reference No. 10 in the 8500 series by Dubuit. The ink is premixed with the hardener referenced 8599 from the same company with a weight content of 10%. Next, the ink is cured at 80xc2x0 C. for 30 minutes. Having deposited the ink, the glazing then undergoes the xe2x80x9cHeat Soak Testxe2x80x9d treatment.
During the treatment, the colour of the ink, which initially was yellow, turns brown. The variation in the colour of the ink deposited thus makes it possible for the glassmaker on the one hand, but essentially for the user on the other, for example in the building industry, to recognize glazing that has undergone the xe2x80x9cHeat Soak Testxe2x80x9d treatment and that has no risk of breaking over time.
In addition, the inventors have carried out precise measurements of the colour using the L*,a*,b* chromaticity coordinates.
These measurements were performed, under illuminant D65 and at a viewing angle of 10xc2x0, using a spectrocolorimeter sold by Minolta with the reference CM 2002.
The measurements were carried out on specimens of glass deposited on a black background, on that face on which the ink is deposited.
The thickness of ink deposited on the specimens was 12 microns.
The table below gives the various measurements carried out on specimens that have undergone a heat treatment up to 300xc2x0 C., with various residence times (expressed in minutes) at 300xc2x0 C.
The start gives the value of the parameters before the heat treatment.
The second table below gives the measured values of the L*,a*,b* coordinates for specimens that have undergone a heat treatment not corresponding to the xe2x80x9cHeat Soak Testxe2x80x9d treatment and therefore at variable temperatures reached and also variable temperature hold times.
The inventors have also measured the variations in another optical characteristic, namely the optical density, during the xe2x80x9cHeat Soak Testxe2x80x9d treatment.
The table below gives the optical density measurements carried out on specimens that have undergone a heat treatment at 300xc2x0 C., with a variable residence time (expressed in minutes) at 300xc2x0 C.
As previously, the start gives the optical density value before the heat treatment.
The final table, below, gives the optical density measurements for specimens that have undergone a heat treatment not corresponding to the xe2x80x9cHeat Soak Testxe2x80x9d treatment and therefore to variable temperatures reached and also variable temperature hold times.